3D Dual Heterostructure Based on MoS2 Modified ZnO

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C: Energy Conversion and Storage; Energy and Charge Transport 2

1D/2D/3D Dual Heterostructure Based on MoS Modified ZnO Heterojunction Diode With Silicon MESWA HARSHADKUMAR PATEL, Pratik Pataniya, Hitesh Vala, and Challappally Kesav Sumesh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05134 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019

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The Journal of Physical Chemistry

1D/2D/3D Dual Heterostructure Based on MoS2 Modified ZnO Heterojunction Diode with Silicon Meswa Patel1, Pratik Pataniya2, Hitesh Vala1, CK Sumesh1* 1Department

of Physical Sciences, P D Patel Institute of Applied Sciences, Charotar University of Science and Technology, CHARUSAT,Changa,Gujarat,India 2Department of Physics, Sardar Patel University, Vallabhvidyanagar,Gujarat,India *[email protected] Abstract Two dimensional (2D) transition metal dichalcogenides (TMDCs) and their composites with metal oxides showed promising applications for visible-light responsive photocatalysis. In this work, we have synthesized optically tunable MoS2.ZnO heterostructure to cover the longer wavelength range in the visible-light region. The optical band gap tuning of the wide band gap ZnO from 3.23 eV to 2.91 eV is successfully achieved via chemical exfoliation and microwave assisted synthesis rout using MoS2 nanosheets. The synthetic heterostructure MoS2.ZnO was prepared to fabricate Si/MoS2.ZnO heterojunction diode, which exhibits a diode like characteristics with excellent photoresponse behavior. It was measured a photoresponsivity of 212.2 mA/W, detectivity of 1.3 × 1010 Jones and response time of 200 ms up on irradiation of 20 mW/cm2 at a bias voltage of -2 V. The overall results are showing that it has potential for large area preparation of optoelectronic and photovoltaic devices. 1. INTRODUCTION Two dimenssional (2D) semiconductor based few-to-monolayer transition metal dichalcogenides (TMDCs) and its hybrids with other 2D/1D (one dimensioanl)/3D(three dimensional) nanomaterials in the form of heterostructure have recently attracted challenging applications due to their unique structures and superior properties that none of the individual conventional 2D nanomaterials could have1,2.

Among the large family of 2D semiconductor TMDCs,

Molybdenum disulfide (MoS2) is a prominent 2D layered material, composes of Mo atoms sandwiched between two layers of hexagonally close packed sulfur atoms. The peculiar layered structure and the thickness dependent indirect to direct band gap nature, MoS2 has come up with

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variety of applications particularly in lithium battery3, solid lubricant4, sensors5, hydrogen production

6–8and

photocatalytic applications7,9–12. The graphene-like structure and direct band

gap at the few-to-monolayer structure captures visible-to-near-IR photon energies helps to use in many optoelectronic applications13,14. However, in its pristine form as single semiconductor, MoS2has drawbacks to use for commercial practical applications in the above mentioned fields because of the intrinsic defects15.

To overcome, it is desirable to design composites and

heterostructures and limit the inherent defects.

Combination of photocatalytic materials (ZnO,

TiO2, CuO, CdS etc.) as co-catalyst with pristine MoS2 can enhance electrochemical performance for photocatalytic applications as well as battery applications6,16–18. These hybrid nanomaterials modify optical, electronic and magnetic properties leads to their wide-range of applications in photocatalytic environmental remediation, solar energy conversion and battery applications.

Xu Zonget al. reported an enhanced photocatalytic activity for the junctions

formed between MoS2 and CdS19. Ying Liu et al. also reported MoS2/CdS heterojunction film for an improved photoelectrochemical performance using visible light20.

An improved

photocatalytic activity and photocurrent response due to enhanced visible light absorption by TiO2/MoS2 composites was proposed by Lingxia Zheng et al. with significantly enhanced photoactivity and improved photocurrent response due to the higher light absorption at visible range and faster charge separation with lower recombination chance comparing to the pristine TiO221.

Therefore, it would be very interesting to study MoS2 based hybrid heterostructures

with suitable co-catalysts for its enhanced capturing visible region of the sunlight. However, the design of hybrid heterostructures to achieve high quality interface with tunable electronic properties from highly mismatched semiconductors are complex and challenging. The weak interlayer van der Waal (vdW) forces in the MoS2 based TMDCs is unique and helps to form stable hybrid structures. There are number of methods to design and preparation of TMDCs hybrid heterostructures studied over last couple of years22–24. Still it needs lot of optimizations to understand the influence of various parameters involved in the synthesis to have consistency, reproducibility, and production yields of high quality composites.

Looking to the group of

metal oxide nano-photocatalyst, the Zinc Oxide (ZnO) has proved one of the promising n-type semiconducting materials due to its wide band gap (~3.37eV, direct band structure, high exciton binding energy (~60 meV), high electron mobility, good thermal and chemical stability, low cost, nontoxic and diverse morphologies25.

These superior properties of ZnO made it as a


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multifunctional material for variety of applications including in optoelectronics, photocatalyst, sensors and many other nanotechnology applications. Owing to the advantages, ZnO has also been used for making composites with other semiconductor including TiO2, ZnS, CdSe, Cu2O, CdS etc. A nanocomposite of ZnO with noble metals as co-catalyst is also an efficient method to improve the catalytic activity. However, one has to check the cost and abundance of the material to be used for commercial applications 26,27. In the family of TMDCs, MoS2 is found to be highly stable, earth abundant, nontoxic and low cost to be used with ZnO as co-catalyst28. Further the conduction band minimum (CBM) of MoS2 nanoparticles is lower than that of ZnO, electrons can spontaneously transfer to CBM of MoS229,30. This will possibly enhance the photoresponse kinetics of photodiode.

Herein, we

have used a facile two stage process to synthesize MoS2 modified ZnO heterostructures by chemical exfolaition and microwave assisted synthesis rout.

We found that the band gap of

MoS2/ZnO (MZO) hybrid structure is tunable with the addition of MoS2 and enhancement in the photoresponse behavior compared to the pristine semiconductor counterparts. The prepared MoS2.ZnO heterostructure was further used to fabricate a dual junction Si/MoS2.ZnO heterojunction diode to study the electro-optical analysis. It was noticed that the Si/MoS2.ZnO heterojunction diode exhibits a diode like behaviour with an enhanced photoresponse parameters with a photoresponsivity of 212.2 mA/W, detectivity of 1.3 × 1010 Jones and response time of 200 ms was observed up on irradiation of 20 mW/cm2 at a bias voltage of -2 V. The adopted methodology and experimental technique involved is less complicated, low cost and be employed for large area synthesis of devices without much precaution. It also can be extended to other 2D family semiconductors and metal oxides for variety of combinations to have novel devices. 2. EXPERIMENTAL 2.1Synthesis of the MoS2–ZnO nanocomposite All the chemical reagents were analytical grade and were used as it was received without further purification.A facile chemical exfoliated combined microwave assisted method was used to synthesize MoS2.ZnO nanocomposites. For synthesis of MoS2 nanosheets, 0.5 gram of bulk MoS2 crystals mixed well with 0.8 ml NMP and was grinded continuously in a mortal pestle for



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30 minutes. The grinding process left with thick dark black slurry. This slurry was annealed in the open air furnace for an hour at 200ºC for drying as reported else where24. The obtained fluffy powder of MoS2 was exfoliated in 20 ml NMP by ultrasonication for 3 hours using Life Care Probe sonicator, 40 kHz, 200W. The as prepared nanosheets of MoS2 was added in 30mM Zinc acetate dehydrate Zn(CH3COO)2·2H2O with ammonia solution (NH4OH) to achieve 10 pH of the solution.

The mixture was sonicated further for 20minutes to form a homogeneous

suspension. The prepared suspension was irradiated in a microwave oven for 15 min set at 360 W. The black precipitates formed after the reaction were washed using methanol and acetone and dried at 150°C for 4 hours to obtain the MoS2.ZnO nanocomposite. The experiment was repeated subsequently by changing the morality of ZnO such as 50mM and 70mM at 30mM mole fractions of MoS2, which is fixed throughout the experiment. The samples were named as MZO(30), MZO(50) and MZO(70) respectively for 30mM, 50mM and 70mM of ZnO. All the samples were prepared under identical conditions and used further without any modifications. 2.2 Fabrication of Si/ MoS2.ZnO Heterojunction diode A p-type silicon wafer (1cm × 1cm) was treated in piranha solution for 1 h, washed with deionized water and methanol for several times, and then dried on a hot plate at 70 °C. MZO nanocomposite dispersion was then drop caste onto Si-wafer. The nanocomposite coated wafer was heated on a hot plate at 130 °C for 10 minto remove solvent. The final structure was annealed in vacuum at 2500C to ensure the removal of trashes of solvents. The in-plane and across plane Ohmic contact with the device was taken with graphite paste and annealed the contact 50oC for 3 hours. 2.3 Characterization The crystal structure and phase constitution of the MoS2, ZnO and MZO heterostructure was characterized by X-ray diffraction (XRD) (D2 phaser BRUKER, M

/KN=0.15418

nm). The

morphologies and microstructure of the sample were examined using scanning electron microscopy (JEOL, JSM 7100F), Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) (JEOL, JEM 2100). The UV absorption spectra of the sample were performed on a UV-VIS spectrophotometer (Shimadzu UV 3600, Japan). The Raman spectrumwas obtained using (Jobin–Yvon, HR800) at room temperature in an ambient air. The identification of functional groups was performed on a FT-IR (Thermo Scientific)


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spectrometer. Current-voltage analysis was done using Keithley-2611 source measurement unit under ambient conditions. 3 RESULTS AND DISCUSSIONS Figure 1 (a) represents the powder X-ray diffraction pattern of the as-obtained MoS2.ZnO heterostructure nanocomposite, which are used to determine the crystallinity, structure and phase of the synthesized nanostructures.

(b)

(a)

Figure1. (a) XRD pattern MoS2.ZnO heterostructure; (b) FTIR transmittance spectra of ZnO, MoS2 and MoS2.ZnO heterostructure The diffraction peaks can be well indexed as those of the hexagonal phase of MoS2 and pure wurtzite structure of ZnO consistent with the standard XRD pattern of ZnO (JCPDS Card No. 73-108) and MoS2 (JCPDS Card No. 89-1397) without the formation of any secondary phases. It has been identified for ZnO that the reflections are observed at 31.29(100), 33.81 (002), 35.70 (101), 46.96 (102), 56.01 (110), 62.30 (103), 65.79 (200), 67.46 (112) ,68.53 (201) and for MoS2,the reflections are at 13.94 (002), 28.49 (004), 31.32 (100), 32.20(102), 39.17(103), 43.75 (006), 49.40 (105), 58.03 (110) and 59.81 (008) represents the pure phase of the prepared samples16,31–34.

The diffractogram of the MoS2.ZnO nanocomposite presents the major

reflections corresponding to ZnO nanoparticles and MoS2nanosheets, revealing the presence of both components.The stacked MoS2 appears in sheet-like structure gives poor signal reflection due to lower atomic packing density and only the (002) characteristic peak is found to be



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dominant in the nanosheets form. The intensity and sharpness of the (002) peak in the MoS2.ZnO nanocomposite structure is increased due to higher crystallinity and stacking of the layers during microwave irradiation. It is also noted that, the XRD patterns of ZnO in MoS2.ZnO nanocomposite are the same diffraction peaks as those of pristine ZnO, but are slightly broader with slightly higher #Q angles. This broadening of peaks in composite samples indicates the decrease in crystallinity due to the formation of MoS2.ZnO heterojunctions.

Further the

incorporation of MoS2 nanosheets with ZnO nanoparticles, there are no major changes on the diffraction pattern of MoS2 and all the major peaks that belong to ZnO nanoparticles are clearly observed. This obviously indicates that MoS2.ZnO nanocomposite is formed as a binary heterostructure-crystalline without the formation of alloy as the homogeneous alloying can cause shifts in the diffraction pattern 13,29,33,35. The presence of various functional group in MoS2.ZnO nanocomposite was analyzed by FTIR spectroscopy. Figure 1(b) displays the FTIR spectra of pristine ZnO, MoS2 and MoS2.ZnO heterostructure. The peaks at 1405, 1639, and 3353 cmR presented in the FTIR pattern of pristine ZnO nanoparticles are indicative for the presence of ZnO36,37. The characteristic FTIR peaks of the MoS2.ZnO heterostructure were observed at 435, 672, 858, 1397 and 3398 cmR . A peak of Mo-S stretching vibration mode was found at 435 cmR . A peak at 672 cmR is ascribed to the asymmetric vibration of the Mo-O group. The peaks 858 cmR revealed the existence of stretching and bending vibration modes of Zn–O bond, respectively38.Hence, from the FTIR spectrum of MoS2.ZnO it can be concluded that MoS2 is coupled well with ZnO in the form of MoS2.ZnO heterostructure. Figure 2 (a) shows the micro-Raman spectra of MoS2.ZnO nanocomposite, whichreveals the presence of various vibrational modes present at respective wavenumbers. The peaks at wavenumbers 377cm-1 (E12g Vibrational mode) and 404 cm1(A

1gVibrational

mode) corresponds to few layer MoS239. The vibrational modes at 331 cm-1, 381

cm-1 and 436 cm-1thatrepresent (E2H-E2L), (A1T) and (E2) vibrational modes respectively for the ZnO Wurtzite structure. Figure 2 (b) presents the UV-Vis. absorption spectra of MoS2.ZnO heterostructure dispersions. The characteristic absorption bands appear approximately at 674, 614, 486 and424nm and are referred to as the A, B, C and D peaks, respectively. The A and B peak arises from direct excitonictransitions at the K point of the Brillouin zone. The C and D peaks are assigned to the direct excitonic transition of M point.


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(a)

(b)

Figure 2. (a) Raman spectra of MoS2.ZnO heterostructure; (b) Optical absorption spectra of MoS2.ZnO heterostructure The absorption edge at 426 nm is indicative of ZnO nanoparticles40,41. Figure 3 (a) shows the topography of MoS2.ZnO composite structure has nanoflakes like structure with densely packed aggregates the MoS2 and ZnO. Energy-dispersive X-ray spectroscopy (EDAX) was carried out to investigate the composition of the prepared samples, and the obtained micrographs are shown in Figure 3 (b). The EDAX analysis of the MoS2.ZnO heterostructure revealed the presence of Zinc (Zn), Molybdenum (Mo), Oxygen (O), and Sulfur (S) in significant concentrations. The morphology of the as synthesized MoS2.ZnO heterostructure was studied using scanning electronmicroscopy (SEM)42.

(1 m)

(a)

(b)

Figure 3. (a) SEM image of MoS2.ZnO heterostructure and (b) EDAX pattern of MoS2.ZnO heterostructure



To further evaluate the surface morphology, high resolution transmission electron microscopy (HRTEM) analysis of the sample was performed and the images are presented in Figure 4 (a). TEM analysis revealed the hexagonally structured MoS2 and ZnOwhere the ZnOis deposited on the layers of MoS2 and the formation of MoS2.ZnO composite structure. It can also be noted that there is an intimate interface between MoS2.ZnO heterojunctions. The selected area electron diffraction (SAED) displayed the ring sport pattern of the hexagonally structured MoS2 and ZnO with an interplanar distance of 0.27 nm and 0.23 nm for (100) and (101) as shown in figure 4 (b).These images show the intimate interface of MoS2.ZnO heterojunctions43.

Figure 4. (a) HRTEM images of MoS2.ZnO heterostructure and (b) SAED patterns MoS2.ZnO heterostructure Optical diffuse reflectance spectra obtained from the UV-Vis-NIR reflectance spectrum was used to calculate the energy band gap of as prepared ZnO and MoS2.ZnO heterostructure. It was noted that, the pure ZnO has an absorption maximum at 382 nm, corresponding to the electronic transitions from the valence band to the conduction band. Tauc’s method was used to find the optical band gap (Eg) of the samples as shown in figure 5 (a) and (b). Here, the plot of +N T,n against hv shows a linear region just above the optical absorption edge, where n = 2 is used for the direct allowed transition, n = 1/2 for the indirect allowed transition, and hv is the photon energy (eV). The band gap Eg is derived from the intercept of this straight line with the photon


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energy axis at +N T,n =0. We estimated the band gap of as synthesized ZnO nanoparticles of 3.23 eV.The effect of loading of MoS2 with ZnO in the band gap of the MoS2.ZnO nanocomposite heterostructure was found to be Eg = 2.91eV at equal mole fractions of 30mM.

(a)

(b)

Figure 5. (a) Ban gap energy of as prepared ZnO; (b) Ban gap energy of MoS2.ZnO(30) heterostructure This study clearly indicates that, MoS2.ZnO nanocomposite heterostructure significantly enhance light absorption in the visible region. Therefore, the UV and visible component of the sunlight can be utilized for the photo-electrochemical applications. The light absorption ability of the MoS2.ZnO increases with the addition of MoS2 as a red shift in the absorption edge of ZnOto a longer wavelength in the visible-light regionwas observed. Hence, it can be concluded that, incorporation of MoS2nanoflakes with ZnO nanoparticles has resulted in the formation of an efficient MoS2.ZnOheterostructure with improved optical properties for opto-electronic applications. The current voltage analysis of the prepared Si/MoS2.ZnOheterostructure diode was studied and confirmed the diode like behavior, as shown in figure 6 (a). The response of the diode was tested under dark and light with reproducible results over multiple runs and it is found that the characteristic is stable. For the I-V measurement, the p-type substrate was kept at positive biasing top MZO contact was given negative to make the junction forward biased for the across plane configuration. To study the effect of influence of the MoS2.ZnOnanocpmposite, on top of




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Is

AA*T 2 exp

q B k BT

(3)

Where, A is the diode area, A* is the Richardson constant and

B

is the barrier height. The

junction parameters are tested for the in-plane MoS2.ZnO heterostructure and for the across plane Si/MoS2.ZnO heterojunction diode under dark and light.

(a)

(b)

Figure 7. (a) lnI-V characteristics Si/MoS2.ZnO across plane diode under dark and light heterostructure diode under dark and illumination; (b)

lnI-V characteristics In-plane MZO

heterostructure under dark and illumination The in-plane characteristics is named as IN(Dark) and IN(Light) respectively for I-V analysis under dark and light as shown in figure 7 (a) and (b).

Similarly for the

Si/MoS2.ZnOheterojunction it is named as A(Dark) and A(Light). For both the heterojunctions the diode factor was found to be far from ideal nature (n~8) indicating the existence of interface defects states44,45. However, the photo-to-dark current ratio of ~761 is achieved for the across plane configuration. For in-plane it was found to be only 10. The barrier heights (BH) in both the cases are approximately 0.7 and 0.72 respectively for across and in-plane. The performance of the heterojunction and the diode parameters influences the factors like the series and parallel resistance arises because of the poor contact at the semiconductor-to-semiconductor junction, leakage current at the interface etc.46–48. The departure from the ideal diode behavior also me be originated from defects due to non-crystallinity, impurity and surface inhomogeneties and complexity of the interfaces of semiconductor junctions. When the negative voltage applied to the silicon side, the depletion region increases gradually and therefore, the barrier potential



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across the junction also increases. Now under light irradiation, the photogenerated electron – hole pairs at the Si/MoS2.ZnOinterface depleted from the junction by the built in potential. Hence, the excitons suppressed under zero bias suddenly increases under reverse bias. The ZnO layers did the light capturing role and MoS2channelize low resistance path for the electron due to its high mobility and large surface area to volume ratio. The conduction mechanisms behind the operation of Si/MoS2.ZnOheterojunction diode and MoS2.ZnOheterostructurecan be explained based on band alignment at the p-Si/n-MZO interface. Many researchers reported the formation of a type-II band structure between Si/MoS2 and MoS2/ZnO theoretically as well as through experiment30,48. Both ZnO and MoS2 exhibit n-type conductivity, the p-n junction formed with p-type silicon causes enhancement in the barrier potential.

During the illumination, the

photogenerated electrons in ZnO can roll down to the conduction band of MoS2, while the photogenerated holes in the WS2 layer may move to the VB of the ZnO layer. Hence the excitons can be effectively transferred to MoS217,48–52. Further, due to the built-in electric field at the Si/MoS2.ZnOheterojunction interface, the recombination of the excitons is suppressed and the intensity of photocurrent enhances compared to dark current.

The

observed photo-to-dark current ratio in the in-plane configuration is negligible and therefore, it could be concluded that MoS2.ZnOheterostructure forms interface with silicon to form a photodiode. However, it is possible that individual heterojunction with Si/ZnO or Si/MoS2 is also possible for the contribution of the photocurrent. The charge transport mechanism also can be explained based on band bending at the Si/MoS2.ZnO heterojunction interfaceas shown in figure 6(b). When light falls on the MoS2.ZnO heterostructure, the generation of excitons occurs because both MoS2 and ZnO absorb the photon energy and electrons move from valance to the conduction band. Further, electrons from the conduction band of ZnO starts flowing to the conduction band of MoS2due to the band alignment. There may be injection of electrons due to thermal agitation and electrons can move from ZnO to conduction band of MoS2.

Thus, the

enhancement in the photocurrent occurs due to the overall injection of electron from ZnO to MoS2 surfaces without much recombination 53.


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dependent variation in detector parameters such as photocurrent, photoresponsivity and detectivity. Furthermore, time resolved photoresponse of Si/MoS2.ZnO heterojunction under white light of power intensity 20 mW/cm2 at different bias voltage from -2 to 2 V was investigated to study the photoresponse behavior. A shown in Figure 8 (a-e), the device exhibits excellent switching between dark and illuminated states of device. The current is steeply enhanced as the device is illuminated and current regains its base current upon termination of illumination. It is attributed to the effective separation upon illumination and recombination of charge carriers in dark condition. This is achieved due to modified properties of MoS2.ZnO nanocomposite and the interface formation with Si with an optimized heterojunction formation.The current is changes from -9.31 V to 32.48 V and -4.34 V to 22.31 V at bias of -2 and -1 V, respectively. Whilst, current in enhanced from 0 V

to 0.11 V

7.09 V

to 31.9 V

and 16.7 V

to 101.6 V

at bias

voltage 0, 1 and 2, respectively. The photocurrent (Iph = Iill-Idark, where, Iill is the current in illuminated condition and Idark is the current is dark condition) is also evaluated. The photocurrent is plotted in Figure 8 (f). The maximum photocurrent of 84.9 V is observed at 2 V bias. Afairly good photoresponse at zero bias is attributing to the fact that the Si/MoS2.ZnOheterojunction shows effective photovoltaic activity upon illumination. It also confirms the type-II interface between MoS2.ZnOnanocomposite and Si substrate. One of the crucial parameter of photodetector is the response time. The equal value rise and decay time of 200 ms depicts excellent charge transport characteristics of Si/MoS2.ZnOheterojunction. However, moderately slow response of device attributes that the performance of Si/MoS2.ZnOheterojunction is limited due to higher density of charge trapping centers. The disorder such as inhomogeneity of junction anddislocations present in nanocomposite leads to higher concentration of deep level defect state which in turn limits the device performance by


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reducing photocurrent and/or prolonging response time54. Besides, the oxygen assisted mechanism55,56can also greatly affect the working on device as the MoS2.ZnO nanocomposite exceptionally high surface to volume ratio. For quantitative analysis of photodetector based on Si/MoS2.ZnO heterojunction, the typical detector parameters such as photoresponsivity (R) and specific detectivity (D) are evaluated using equation (4) and (5), respectively57.

R

D

I ph PS R S 1/ 2 1/ 2 2eI dark

(4)

(5)

where, Iph is the photocurrent, S is the effective area of the photo detector (~0.02 cm2), P is the illumination intensity (20 mW/cm2) and e is the elementary electronic charge (1.6×10-19 C). The detector parameters are portrayed in Figure 8 (f) against bias voltage. The maximum photoresponsivity of 212.2 mA/W and detectivity of 1.3 × 1010 Jones are observed at -2 V bias. There are few reports on Si-MoS2 based photodetectors and ZnO-MoS2 photodetectors, which revealed excellent photoresponse properties. Qiaoet al. demonstrated a vertical layered MoS2/Si heterojunction photodetector synthesised by chemical vapour deposition (CVD) technique. The device showed a very wide photoresponse ranging from 350 to 1100 nm with photoresponsivity and detectivity up to 908.2 mA/W and 1.889x1013 Jones, respectively58. Shin et al. demonstrated mechanically exfoliated multilayer MoS2/Si heterojunctions with with a responsivity and detectivity of 76.1 A/W and 1012 Jones, respectively59. Haoet al. used magnetron sputtering technique to prepare Pd-MoS2/Si p-n junction diode with detectivity of 1014 Jones and a responsivity of 654 mA/W60. Naziret al. adopted a mechanical exfoliation using the scotch tape method to prepare ZnO quantum dot decorated MoS2 thin films photodetectors with photoresponsivity and detectivity up to 1913 mA/W and 1011 Jones, respectively53. However, there are only very few studies on MoS2 heterostructure photodiodes based on chemical synthesis rout. Mukherjee et al. demonstrated a chemically exfoliated MoS2/Si heterostructure interface with a responsivity of 470 mA/W 49. The results are comparable with the similar works



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reported on Si/MoS2 and ZnO/MoS2 based photodetectors mentioned above. However, in the present case, we have adopted a facile and low cost preparation technique prepare MoS2 nanosheets and its heterostructures. Therefore, the method adopted here can be exploited to make diode with large area substrates. 4 CONCLUSIONS In summary, we have synthesized MoS2.ZnO nanocomposite heterostructure using a twostagesonochemical exfoliation and microwave assisted synthesis method.

The structural and

morphological analysis indicates that the prepared MoS2.ZnOformed abinary heterostructurecrystalline structure.The optical absorption and reflection study revealed the interface interaction between MoS2 stacks and ZnO nanoparticles reduces the band gaps of MoS2.ZnO nanocomposite heterostructure compared to pristine ZnO. Hence, the electronic properties of the ZnOcan be tuned to explore more towards the visible region of the solar spectrum by adding MoS2 in stoichiometric proportion. Compared to pure ZnO, MoS2.ZnO nanocomposite heterostructure has a smaller band gap of 2.91eV, extending the absorption spectrum covering to a longer wavelength in the visible-light region. The prepared Si/MoS2.ZnO heterojunction diodes show a diode like behavior with an excellent photoresponse property.

It was measured a

photoresponsivity of 212.2 mA/W, detectivity of 1.3 × 1010 Jones and recovery times of 200 ms up on illumination of 20 mW/cm2 at a bias voltage of -2 V. The overall results show that the Si/MoS2.ZnO heterojunction diode in the form of dual junction can be one of the good candidates for the large area photovoltaic devices. ACKNOWLEDGEMENT The authors are immensely grateful Central Salt & Marine Chemicals Research Institute (CSMCRI) for providing TEM and SEM data. The research facility provided by the CHARSAT University is gratefully acknowledged. REFERENCES (1) Tang, K.; Qi, W.; Li, Y.; Wang, T. Electronic Properties of van Der Waals Heterostructure of Black Phosphorus and MoS2. J. Phys. Chem. C 2018, 122 (12), 7027–7032.


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TOC Graphics



(b)

(a)

Figure1. (a) XRD pattern MoS2.ZnO heterostructure; (b) FTIR transmittance spectra of ZnO, MoS2 and MoS2.ZnO heterostructure

(a)

(b)

Figure 2. (a) Raman spectra of MoS2.ZnO heterostructure; (b) Optical absorption spectra of MoS2.ZnO heterostructure


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(1μm)

(a)

(b)

Figure 3. (a) SEM image of MoS2.ZnO heterostructure and (b) EDAX pattern of MoS2.ZnO heterostructure

Figure 4. (a) HRTEM images of MoS2.ZnO heterostructure and (b) SAED patterns MoS2.ZnO heterostructure



(a)

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(b)

Figure 5. (a) Ban gap energy of as prepared ZnO; (b) Ban gap energy of MoS2.ZnO(30) heterostructure Ohmic contact

MZO P-Si

Ohmic contact

+ ve . .

. .

Ec

- ve

EF EV

. . . .

.

hν

.

Ec

hν. . .

EF EV

. . .

. . . . .

P-Si

n-MZO Depletion layer

(b)

(a)

Figure 6. (a) I-V characteristics Si/MoS2.ZnO heterostructure diode under dark and illumination; (b) Band bending diagram of Si and MZO interface


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(a)

(b)

Figure 7. (a) lnI-V characteristics Si/MoS2.ZnO across plane diode under dark and light heterostructure diode under dark and illumination; (b) lnI-V characteristics In-plane MZO heterostructure under dark and illumination



Figure 8. (a-e) Time resolved photoresponse of Si/MoS2.ZnOheterojunction at different bias ranging from -2 to 2 V under white light of 20 mW/cm2 power intensity and (f) Voltage dependent variation in detector parameters such as photocurrent, photoresponsivity and

detectivity.


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3D Dual Heterostructure Based on MoS2 Modified ZnO

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